Coating system

ABSTRACT

The present invention provides a coating system comprising an antireflective functionality and UV absorbing functionality. The present invention further provides methods, uses, and articles comprising such a system.

The present invention relates to coating systems. In particular thepresent invention relates to coating systems with low reflectivity andlow transmission of ultra-violet (UV) radiation.

UV radiation has a deleterious effect on a wide variety of materials.For example, it can cause yellowing of materials and/or fading ofcolours. This is a particular issue when the item being exposed is ofhigh value, such as with artwork, but is also a problem for more mundaneitems such as drapes, carpets, wallpapers and the like. In addition,certain materials are degraded by UV radiation.

UV control films are known. See for example U.S. Pat. No. 4,275,118,U.S. Pat. No. 4,455,205, U.S. Pat. No. 4,799,963, and EP0732356. Thereare also commercially available films based on organic UV absorbers.

The present invention provides a coating system comprising anantireflective functionality and UV absorbing functionality. The presentinvention further provides methods, uses, and articles comprising such asystem.

As used herein, the term “nano-particles” refers to colloidal particleswhose primary particle size is less then 1 μm, preferably of less than500 nm, more preferably of less than 350 nm.

As used herein, the term “binder” refers to a substance that canchemically cross-link the particles and preferably also between theparticles and a substrate.

As used herein, the term “pre-hydrolysing” refers to hydrolysing themetal alkoxide binder precursor to the point that oligomeric species areproduced via partial condensation but not to the point that gelationoccurs.

Unless otherwise stated all references herein are hereby incorporated byreference.

In one embodiment, the present invention comprises a substrate, acoating layer comprising particle cerium, titanium or zinc oxide or acombination thereof, and a coating layer comprising nano-particles of ametal oxide.

Any suitable substrate may be used herein. Preferably the substrateallows transmission of light in the visible and UV spectra. Preferablythe substrate is transparent or translucent. The substrate preferablyhas a high transparency. Preferably the transparency is about 94% orhigher at 2 mm thickness and at wavelength between 425 and 675 nm, morepreferably about 96% or higher, even more preferably about 97% orhigher, even more preferably about 98% or higher.

The substrate herein may be organic. For example, the substrate may bean organic polymeric such as polyethylene naphthalate (PEN),polycarbonate or polymethylmethacrylate (PMMA), polyester, or polymericmaterial with similar optical properties. In this embodiment, it ispreferred to use a coating that can be cured at temperaturessufficiently low that the organic material remains substantially in itsshape and does not suffer substantially due to thermal degradation. Onepreferred method is to use a catalyst as described in EP-A-1591804.Another preferred method of cure is described in WO 2005/049757.

The substrate herein may be inorganic preferably glass or quartz.Preferred is float glass. Generally, a glass plate has a thickness of0.5 mm or more, preferable 1 mm or more, most preferably, about 1.8 mmor more. Generally, the glass plate has a thickness of about 20 mm orless, preferably about 10 mm or less, more preferably about 6 mm orless, more preferable about 4 mm or less, and most preferred, about 3 mmor less.

The system of the present invention comprises a UV protective layer.This layer is preferably applied directly to the substrate. This layercomprises particles of cerium oxide, titanium oxide, zinc oxide, orcombinations thereof. Preferably the layer comprises particles of ceriumdioxide, titanium dioxide, zinc oxide, or combinations thereof.Preferably the layer comprises particles of cerium oxide, morepreferably cerium dioxide.

Surprisingly, it has been found that the layer is much more stable andeasier to work with when the pH of the coating composition iscontrolled. Preferably the pH is below about 6, more preferably belowabout 5.5.

Preferably the UV protective layer comprises a binder. Preferably thebinder forms covalent bonds with the particles and the substrate. Forthis purpose, the binder—before curing—preferably comprises inorganiccompounds with alkyl or alkoxy groups. Further, the binder preferablypolymerises itself to form a substantially continuous polymeric network.

In one embodiment the binder of the UV layer consists substantially ofan inorganic binder. The inorganic binder is preferably derived from oneor more inorganic oxides. Preferably the binder is a hydrolysablecompound such as metal-alkoxides. Preferably the binder is selected fromalkoxy silanes, alkoxy zirconates, alkoxy aluminates, alkoxy titanates,alkyl silicates, sodium silicates, and mixtures thereof. Preferred arealkoxy silanes, preferably tri and tetra alkoxy silanes. Preferably,ethyl silicate, aluminate, zirconate, and/or titanate binders are used.Tetra alkoxy silane is most preferred.

Preferably the binder is ‘pre-hydrolyzed’. That is, the binder hasundergone some degree of hydrolyzation prior to formulating with theparticles.

The reaction of the particles and binder is preferably performed in asolvent, which is preferably a mixture of water and an organic solvent.Depending on the chemistry of the binder, many solvents are useful.Suitable solvents include, but are not limited to, water, non-proticorganic solvents, alcohols, and combinations thereof. Examples ofsuitable solvents include, but are not limited to, isopropanol, ethanol,acetone, ethylcellosolve, methanol, propanol, butanol, ethyleneglycol,propyleneglycol, methyl-ethyl-ether, methyl-butyl-ether, toluene,methyl-ethylketone, and combinations thereof.

The UV-absorption capacity may be increased by increasing theconcentration of particles. However, this can also lead to stabilityproblems due to lack of binder. Therefore, there is always a balance tobe struck between physical performance and UV-absorption. One way ofincreasing the UV-absorption is to add a doping agent. For example,titanium doping agent can be added to the particles.

Preferably the weight ratio of particles to binder in the layer is fromabout 100:1 to about 1:100, More preferably from about 10:1 to about1:10. Even more preferably from about 5:1 to about 1:5.

Preferably the layer has a dry thickness of from about 50 nm to about500 nm. More preferably the layer has a thickness of from about 100 nmto about 250 nm.

Preferably the UV solution is prepared by reacting the particles withbinder and allowing the reaction to proceed until substantiallycomplete. Then further binder is added. Surprisingly this helps avoidthe formation of an undesirable gel and allows for easier coating of thesubstrate.

The layer may be applied to the substrate in any suitable manner.Preferred methods of application include meniscus (kiss) coating, spraycoating, roll coating, spin coating, and dip coating. Preferably thelayer is applied by dipping the substrate in the coating composition andthen removing. A constant withdraw is preferred in order to improve theevenness of the coat. A second coat may be applied for extra UVprotection.

A preferred method of coating herein comprises:

-   -   (i) cleaning the substrate,    -   (ii) dipping the substrate in a solution comprising particles        and binder,    -   (iii) withdrawing the substrate at a substantially constant        rate,    -   (iv) allowing the solvents to evaporate.

The system of the present invention comprises an anti-reflective (AR)layer. The AR layer preferably comprises nano-particles of a metaloxide. Examples of suitable particles include, but are not limited to,particles comprising lithium fluoride, calcium fluoride, bariumfluoride, magnesium fluoride, titanium dioxide, zirconium oxide,antimony doped tin oxide, tin oxide, aluminum oxide, silicon dioxide,and mixtures thereof. Preferably the particles comprise silicon dioxide.More preferably the particles comprise at least 90% by weight of silicondioxide.

Preferably the nano-particles have a length of less than 1000 nm, morepreferably of less than 500 nm, even more preferably of less than 350nm.

In one embodiment the particles preferably have an average aspect ratioat least 1.5. Preferably the average aspect ratio of the particles is atleast 2, more preferably at least 4, even more preferably at least 6,still more preferably at least 8, even more preferably at least 10.Preferably the aspect ratio will be about 100 or lower, preferably about50 or lower.

The sizes of the particles may be determined by spreading a dilutesuspension of the particles over a surface and measuring the sizes ofindividual particles by using microscopic techniques, preferablyscanning electronic microscopy (SEM) or atomic force microscopy (AFM).Preferably the average sizes are determined by measuring the sizes of atleast 100 individual particles. The aspect ratio is the ratio betweenthe length and the width of a particle. In case of rods and worm-likeparticles the length is the largest distance between two points in theparticle and the width is the largest diameter as measured perpendicularto the central axis of the particle. Both length and width are measuredfrom the projection of the particles as observed under the microscope.

The coating AR layer herein may comprise a mixture of different typessizes and shapes of particles.

In one embodiment the particles used herein are non-spherical such as,preferably, rod-like or worm-like particles, preferably worm-likeparticles. Worm-like particles are particles having a central axis thatdeviates from a straight line. Examples of worm-like particles are knownby the tradename Snowtex (IPA-ST-UP), particles have a diameter of 9-15nm with a length of 40-300 nm), available from Nissan Chemical.Hereinafter, rod-like and worm-like particles are also denoted aselongated particles.

In a preferred embodiment the particles used herein are substantiallyspherical. Preferably the spherical particles have an average aspectratio of about 1.2 or lower, preferably of about 1.1 or lower.Preferably the particles have an average size of about 10 nm or larger,preferably 20 nm or larger. Preferably the particles will have anaverage size of 200 nm or smaller, preferably 150 nm or smaller, evenmore preferably about 100 nm or smaller. Substantially sphericalparticles have the advantage that they form coatings where the volume ofnano-pores resulting from the space between the particles is smallrelative to the volume formed by non-spherical particles. Thus thecoatings suffer less from filling of the nano-pores via capillary forceswhich can cause a loss in anti-reflective performance. These particlesmay have a narrow or broad particle size distribution, preferably abroad particle size distribution.

The particles herein are generally provided in a solvent. For example,the solvent may be water or an alcohol such as methanol, ethanol orisopropanol (IPA).

The nano-particles are preferably reacted with a surface modifying agentso that particles are obtained which are reactive with the binder. Thesurface modifying agent(s) react with the nano-particle to cause theparticle to be activated so that it is more effectively able to reactwith the binder. The surface modifying agent is preferably one that isable to form oxides. Preferably, the surface modifying agent is ahydrolysable compound such as, for example, metal-alkoxides. Suitableexamples include, but are not limited to, alkoxy silanes, alkoxyzirconates, alkoxy aluminates, alkoxy titanates, alkyl silicates, sodiumsilicates, and mixtures thereof. Preferably alkoxy silanes, morepreferably tri and tetra alkoxy silanes, are used. Tetra alkoxy silaneis more preferred.

Generally, the reaction is performed in a solvent. Depending on thechemistry of the binder, many solvents are useful. Suitable examples ofsolvents include water, non-protic organic solvents, and alcohols.Examples of suitable solvents include, but are not limited to,isopropanol, ethanol, acetone, ethylcellosolve, methanol, propanol,butanol, ethyleneglycol, propyleneglycol, methyl-ethyl-ether,methyl-butyl-ether, 1-methoxy propan-2-ol, toluene, methyl-ethylketone,and mixtures thereof. Preferred are isopropanol, ethanol, methanol,propanol, and mixtures thereof.

The AR layer preferably comprises a binder. The binder has the primaryfunction of keeping the surface activated particles attached to eachother the substrate. Preferably the binder forms covalent bonds with theparticles and the substrate. For this purpose, the binder—beforecuring—preferably comprises inorganic compounds with alkyl or alkoxygroups. Further, the binder preferably polymerises itself to form asubstantially continuous polymeric network.

In one embodiment of the invention the binder of the coating consistssubstantially of an inorganic binder. The inorganic binder is preferablyderived from one or more inorganic oxides. Preferably the binder is ahydrolysable compound such as metal-alkoxides. Preferably the binder isselected from alkoxy silanes, alkoxy zirconates, alkoxy aluminates,alkoxy titanates, alkyl silicates, sodium silicates, and mixturesthereof. Preferred are alkoxy silanes, preferably tri and tetra alkoxysilanes. Preferably, ethyl silicate, aluminate, zirconate, and/ortitanate binders are used. Tetra alkoxy silane is most preferred.

Preferably the pH of the solution is about 2 or higher, more preferredabout 3 or higher. The pH is preferably about 5.5 or lower, morepreferred about 4.5 or lower.

The nano-particles and binder may be mixed in such a ratio that chosenoptical and mechanical properties are obtained. In addition to theparticles and binder other components may be added, such as furthersolvent, catalyst, hydrophobic agent, levelling agent, and the like. Inone embodiment the present coating compositions comprise:

-   -   (i) nano-particles of a metal oxide,    -   (ii) metal oxide based binder,        wherein the weight ratio of metal oxide in (i) to (ii) is from        99:1 to 1:1. Preferably the weight ratio of metal oxide is from        85:1 to 3:2, more preferably from 65:1 to 2:1.

Preferably the AR layer is applied to the substrate article so that theresultant dry coating thickness is about 50 nm or greater, preferablyabout 70 nm or greater, more preferably about 90 nm or greater.Preferably the dry coating thickness is about 300 nm or less, morepreferably about 200 nm or less.

The AR layer may be applied to the substrate by any suitable means.Preferably the AR layer is applied after the UV protective layer.Preferably the AR layer is applied on top of the UV protective layer.Preferred methods of application include meniscus (kiss) coating, spraycoating, roll coating, spin coating, and dip coating. Dip coating ispreferred, as it provides a coating on all sides of the substrate thatis immersed, and gives a repeatable and constant thickness. Spin coatingcan easily be used if smaller glass plates are used, such as ones with20 cm or less in width or length. Meniscus, roll, and spray coating isuseful for continuous processes.

It was surprising that the present AR layer could be easily applied ontop of the UV protective layer without significantly affecting thefunction of either even without the need for curing (hardening) of theUV protective prior to application of the AR layer. Therefore apreferred embodiment of the present system comprises:

-   -   (i) coating a substrate with the UV protective layer,    -   (ii) coating the AR layer on top of the UV protective layer.

Preferably the present coating system is such that, when measured forone coated side at a wavelength between 425 and 675 nm (the visiblelight region), the minimum reflection is about 2% or less, preferablyabout 1.5% or less, more preferably about 1% or less. The averagereflection at one side, over the region of 425 to 675 nm preferably willbe about 2.5% or less, more preferably about 2% or less, even morepreferably about 1.5% or less, still more preferably about 1% or less.Generally, the minimum in the reflection will be at a wavelength between425 and 650 nm, preferably at a wavelength of 450 nm or higher, and morepreferably at 500 nm or higher.

The mechanical properties can be tested as steel wool resistance.Preferably, the coating system has ‘acceptable’ steel wool resistancewhich is defined as less than 10 observable scratches after 10 rubs with0000 steel wool with a loading of 250 g. More preferably, the steel woolresistance is ‘good’ which is defined 3 or less observable scratchesafter 10 rubs with 0000 steel wool with a loading of 250 g.

Preferably the present system reduces UV transmission through to thesubstrate by 50% or more, more preferably 60% or more, even morepreferably 70% or more.

Preferably at least about 20% or more, preferably about 50% or more,even more preferably about 90% or more, of one of the surfaces of thesubstrate is coated with the present system.

For all coating processes, cleaning is an important step, as smallamounts of contaminant such as dust, grease and other organic compoundscause the anti reflective coating, or other coatings to show defects.Cleaning can be done in a number of ways, such as firing (heating up to600-700° C.; applicable if an inorganic substrate is used); and/orcleaning with a cleaning fluid such as soap in demineralised water,alcohol, or acidic or basic detergent systems. When using a cleaningfluid, generally, the glass plate is dried at a temperature between 20°C. and 400° C., optionally with applying an air flow.

In one embodiment a substrate comprising the present system is used forframing of pictures, photos, paintings, posters, etches, drawings,fabrics, tapestries and the like.

The present system may also be used for applications such as displaycases, architectural glass, solar panels, automotive glass, and thelike.

The invention will be further elucidated by the following examples,without being limited thereto.

EXAMPLES

98 g of tetraethoxysilane (TEOS) were added to 267 g of isopropanol(IPA), 90 g of water and 10 g of acetic acid before being stirred for 72hours at room temperature (RT). This mixture was then diluted with 270 gof IPA and 2 g of concentrated aqueous hydrochloric acid. This formedprehydrolysed TEOS.

91.22 g of IPA was mixed with 31.36 g of Snowtex (IPA-ST-UP-15.6 wt % inIPA) particles, 11.76 g of TEOS and 15.66 g of water. The solution wasstirred for 4 hours at 80° C. Then a further 150 g of IPA was addedalong with 11.5 g of pre-hydrolysed TEOS. This formed the ARformulation.

60 g of ceria particles were mixed with 2.25 g of TEOS and stirred forthree hours. Then a further 4.3 g of TEOS were added with 93.5 g of IPA.This formed the UV formulation.

A glass plate (10 cm×10 cm) was washed and polished before being dippedin the solution of UV formulation for 4 seconds. The plate was removedat a constant speed of 0.33 cm/s. The solvents were allowed toevaporate. The process was repeated three more times. The plate was thendipped in the AR formulation for 4 seconds and withdrawn at a rate of0.2 cm/s.

The resultant plate was cured for 4 hours at 450° C.

The plate showed a 75% reduction in transmission of UV radiation and areflection of 0.42% at 470 nm.

The plate was then tested for abrasion resistance. A flat circular steelsurface (diameter=2.1 cm) was cover evenly with steel wool (grade: 0000)with a normal weight of 250 g. The steel wool was then moved back andforth over the surface 5 times making for a total of 10 rubs over adistance of 5 to 10 cm. At this point the surface of the coating isvisually inspected and rated according to the number of observablescratches. 0-3 scratches gave a rating of A, 4-10 gave a rating of B,11-15 gave a rating of C, 16-30 gave a rating of D, coating completelyremoved gave a rating of E. The plate of this example had a rating of A.

1. A coating system comprising: (i) a substrate (ii) an ultra-violetprotective layer comprising particles of a cerium oxide, a titaniumoxide, a zinc oxide, or mixtures thereof. (iii) an anti-reflective layercomprising nano-particles of metal oxide.
 2. A system according to claim1 wherein the ultra-violet protective layer comprises a cerium oxide. 3.A system according to claim 1, wherein the substrate is selected frompolyethylene naphthalate, polycarbonate or polymethylmethacrylate(PMMA), polyester, quartz, glass, and combinations thereof.
 4. A systemaccording to claim 1 wherein the ultra-violet protective layer comprisesa binder.
 5. A system according to claim 1 wherein the anti-reflectivelayer comprises a binder.
 6. A system according to claim 1 wherein theanti-reflective layer comprises nano-particles of a silicon oxide.
 7. Asystem according to claim 1 wherein the ultra-violet layer is preparedby process comprising the steps of: (i) reacting the particles with abinder, (ii) adding further binder to the pre-reacted particles.
 8. Acoating system comprising: (i) a substrate having a refractive index offrom about 1.4 to about 2.5, (ii) a first coating layer having arefractive index of from about 1.7 to about 1.9, and (iii) a secondcoating layer having a refractive index of from about 1.3 to about 1.5.9. A cured coating system according to claim
 1. 10. Use of a coatingsystem according to claim 1 for providing glass for framing. 11.Articles comprising a coating system according to claim 1.